Health, in human beings, the extent of an individual’s continuing physical, emotional, mental, and social ability to cope with his environment.

This definition, just one of many that are possible, has its drawbacks. The rather fragile individual who stays “well” within the ordinary environment of his or her existence may succumb to a heart attack from heavy shovelling after a snowstorm; or a sea-level dweller may move to a new home in the mountains, where the atmosphere has a lower content of oxygen, and suffer from shortness of breath and anemia until his red blood cell count adjusts itself to the altitude. Thus, even by this definition, the conception of good health must involve some allowance for change in the environment.

Bad health can be defined as the presence of disease, good health as its absence—particularly the absence of continuing disease, because the person afflicted with a sudden attack of seasickness, for example, may not be thought of as having lost his good health as a result of such a mishap.

Actually, there is a wide variable area between health and disease. Only a few examples are necessary to illustrate the point: (1) It is physiologically normal for an individual, 15 to 20 minutes after eating a meal, to have a high blood sugar content. If, however, the sugar content remains elevated two hours later, this condition is abnormal and may be indicative of disease. (2) A “healthy” individual may have developed an allergy, perhaps during early childhood, to a single specific substance. If he never again comes in contact with the antigen that causes the allergy, all other factors remaining normal, he will remain in that state of health. Should he, however, come in contact with that allergen, even 20 or 30 years later, he may suffer anything from a mild allergic reaction—a simple rash—to severe anaphylactic shock, coma, or even death, depending upon the circumstances. Thus it can be seen that, unlike disease, which is frequently recognizable, tangible, and rather easily defined, health is a somewhat nebulous condition, and somewhat difficult to define.

Moreover, physical condition and health are not synonymous terms. A seven-foot-tall basketball player may be in excellent physical condition (although outside the range of normality for height) but may or may not be in good health—depending, for example, on whether or not he has fallen victim to an attack of influenza.

There are further problems in settling upon a definition of human health. A person may be physically strong, resistant to infection, able to cope with physical hardship and other features of his physical environment, and still be considered unhealthy if his mental state, as measured by his behaviour, is deemed unsound. What is mental health? Some say that a person is mentally healthy if he is able to function reasonably well. Others hold that a person is healthy mentally if his behaviour is like that of a majority of his fellows.

In the face of this confusion, it is most useful, perhaps, to define health, good or bad, in terms that can be measured, can be interpreted with respect to the ability of the individual at the time of measurement to function in a normal manner and with respect to the likelihood of imminent disease. These measurements can be found in tables of “reference values” printed in textbooks of clinical medicine, diagnosis, and other references of this type. When an individual is given a health examination, the examination is likely to include a series of tests. Some of these tests are more descriptive than quantitative and can indicate the presence of disease in a seemingly healthy person. Such tests include the electrocardiogram to detect some kinds of heart disease; electromyogram for primary muscle disorders; liver and gall bladder function tests; and X-ray techniques for determining disease or malfunction of internal organs.

Other tests give numerical results (or results that can be assigned numerical values—such as photometric colour determinations) that can be interpreted by the examiner. These are physical and chemical tests, including blood, urine, and spinal-fluid analyses. The results of the tests are compared with the reference values; and the physician receives clues as to the health of his patient and, if the values are abnormal, for the methods of improving his health.

A major difficulty in the interpretation of test results is that of biological variability. Almost without exception these reference values for variables are means or adjusted means of large group measurements. For these values to have significance, they must be considered as lying somewhere near the centre point of a 95 percent range—i.e., the so-called ordinary range or, with reservations, the range from normal to the upper and lower borderline limits. Thus, the 2.5 percent below the lower limit and the 2.5 percent above the upper limit of the 95 percent range are considered areas of abnormality or, perhaps, illness. Some areas have wide 95 percent ranges—blood pressure, for example, may vary considerably throughout the day (e.g., during exercise, fright, or anger) and remain within its range of normality. Other values have ranges so narrow that they are termed physiological constants. An individual’s body temperature, for example, rarely varies (when taken at the same anatomical site) by more than a degree (from time of rising until bedtime) without being indicative of infection or other illness.

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When the World Health Organization (WHO) was established in 1948 as a specialized agency of the United Nations to further international cooperation for improved health conditions, its constitution defined health as "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity" and went on to declare that "enjoyment of the highest attainable standard of health is one of the fundamental rights of every human being." Although some consider WHO’s concept of health "lofty" and others say that health-like beauty-is in the eye of the beholder, that definition has not been amended and is still widely accepted. The Indian political and spiritual leader Mahatma Gandhi, who died in 1948, a few months before WHO began its work of promoting the optimal health of people throughout the world, articulated the value of health as: "It is health that is real wealth and not pieces of gold and silver."

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